The development of improved techniques to manage excess lengths of fiber optic cabling has become particularly relevant with the spread of optical communications technology from the long haul and metro networks to the interconnection rich enterprise and access networks. A unique characteristic of fiber-based transmission media is that considerable care must be taken in handling fiber optic cables because of the potential to damage the internal glass optical fiber. Unlike electrical cables, which can be sharply bent or subjected to significant forces without impacting their performance characteristics, fiber optic cables can easily be damaged under small shear forces and must maintain a minimum bend radius. Sharp bends result in increased insertion loss, stress birefringence (manifested as polarization dependent loss), and ultimately fiber failure. In addition, the interface between the polished fiber optic connector and cable is particularly sensitive to damage arising from the concentration of stress at the connector interface and the weight of the connector body.
The preparation of fiber optic patchcords requires the use of a complex and time consuming polishing process which adds considerable cost relative to electronic cabling. Optical fiber cables can not be readily cut to length in the field, nor can they be simply folded to take up excess length. Unlike electrical connectors, optical connectors are highly sensitive to damage arising from contamination or scratching. This damage results in potential data corruption or complete loss of data transmission. Therefore, techniques to mitigate damage to fiber optic cables address an important problem.
Early attempts to provide optical systems with retractable fiber optic cables, that is, cables wherein excess lengths are automatically wound onto a spool, have been unsuccessful because of the high cost and complexity of the proposed solutions. For instance, the design of communications interfaces with retractable fiber optic cables has been described in U.S. Pat. No. 6,014,713 by Agnew et al. and U.S. Patent Application 2004/0081404 by Elliott et al. Both devices involve a retraction unit in which one cable end is fixed in location, while the other cable end is extendable/retractable. A significant disadvantage of these approaches is that the fiber optic cable is physically interrupted. A collimator pair comprising a rotary junction allows one cable to rotate relative to the other and results in considerable complexity and excessive cost. In particular, the rotatable spool includes a terminated and lensed fiber interface which must remain in precise alignment with a reciprocal, fixed lensed fiber interface to achieve optical continuity.
Various patents describe a wide range of fiber optic rotary joints and slip rings, wherein the optical fiber is cut and lenses and/or optics are placed in alignment to transmit signals or illumination across the rotary interface. U.S. Pat. Nos. 4,109,998, 4,124,272, 4,258,976, 4,447,114, 4,641,915, 4,749,249, 4,872,737, 5,371,814, 5,450,509, 5,442,721, 5,921,497, 6,301,405, 6,453,088, 6,799,878, and Patent Application US 2004/0086222 present different versions of this concept. Significant complexity is added for a rotary interface in which multiple fibers must remain in communication.
In an alternate approach, U.S. Pat. Nos. 5,078,466 and 6,819,854 describe a fiber optic rotary joint comprised of a flexible ribbon cable joining rotating and stationary parts of a structure. These approaches do not address the unique requirements of a low loss fiber optic rotary interface, which requires that a gradual fiber bend radius be maintained. In particular, the combination of friction between cable surfaces and lack of transverse rigidity of fiber is significant enough to severely limit the amount of angular rotation that can be produced while maintaining a minimum bend radius. Adjacent turns bind once they are tightly packed about the inner or outer diameter (corresponding to the wound or unwound configurations). Therefore, this approach is inadequate to prevent binding and to provide a substantial number of rotations in a low loss manner.
An alternate retractable fiber optic cable approach (U.S. Patent Application 2004/0170369) by Pons utilizes a continuous length of fiber which is wound pair-wise about a spool. This approach suffers from the limitation that both fibers must be retracted or extended together. In most applications, it is preferred that one end of the fiber is of fixed length, while the other end is continuously extendable.
Significant optical network performance and cost advantages are derived by the ability to retract a variable length of un-interrupted optical fiber cable in a convenient and cost effective manner. To further maintain low loss and low backreflection, it is important that the retraction approach does not physically interrupt or cut the cable. Furthermore, the rotary interface design must provide in excess of a few turns to enable significant cable lengths to be retracted within a compact housing.